STATUS OF PROJECT TOR FALL 2000

Results are now coming from Project Tor, which is about Teleseismic Tomography across the Tornquist Zone in Germany-Denmark-Sweden. We are able to distinguish very significant deep lithosphere differences, and the sharpness laterally is discussed. Our 120 seismographs constituted in 1996-1997 the Iargest seismic antenna ever in Europe. The Tor area was chosen along a well studied crustal profile of an earlier project, and the inversion efforts are concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km. The Tor investigation can be called two-and-a-half dimensional, being a 900 km profile with 100 km width plus a few seismographs off the profile. It includes several seismic methods besides travel time tomography, surface wave and receiver function analysis as well as anisotropy and scattering measurements. Through ray tracing in a compiled crustal model and subtraction of the modelled travel time anomalies, the influence of the lower lithosphere/asthenosphere on the seismic rays from distant earthquakes is established. Travel-time tomography results confirm very large lithosphere differences. For several events of the large data base it is shown that the observed travel time anomalies of 1-2 seconds can be divided almost equally between known crustal effects and lower lithosphere/asthenosphere differences, which then must account for about one second of travel time differences. The transition is interpreted to be sharp and steep in two places. It goes all way through the lithosphere at the northern rim of the Sorgenfrei-Tomquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large. A smaller lithosphere difference is found at the southern edge of the Ringkøbing-Fyn High just north of the border between Denmark and Germany. Here the transition is similarly sharp and steep, and all through the lithosphere. These two sharp transitions divide the Tor region into 3 different lithosphere structures distinguishable in P-wave travel time tomography, surface wave dispersion, S-wave anisotropy and partly in P-wave scattering.